In what is referred to as a "teleprocessing" operation, a computer is attached to another device, either another computer or a remote terminal, for the transmission of data therebetween. The line interconnecting a computer or remote terminal may be a telephone network, leased lines or the like, and the common practice is to utilize a modem at each location in the transmission network.
In one such teleprocessing operation, an operator at a remote terminal who has data to transmit, generates such data through the remote terminal for transmission to the computer, one character at a time, in what is referred to as a "conversational mode", usually on a key stroke basis. In such a "conversational mode", the operator at the remote terminal begins generating and sending data to the computer, one character at a time, and it is up to the computer program to sense that this data has arrived, interrupt the program that it is performing, and process the data, one character at a time.
In the more widely used type of teleprocessing operation, what is referred to as "polled network", the operator first composes a complete message for the computer. During the time the operator is composing the message, the computer to which the message is to be sent is processing its regular program, but it stops occasionally and sequentially addresses all of the remote terminals to which it is connected to determine if any of the remote terminals has data which is to be sent. If a remote terminal has a message ready for transmission, the operator depresses a transmit key and the next time the computer polls the network, the prepared data will automatically be sent to the computer. It is this type of "polled network" that the modem sharer of the invention is directed, as most large commercial teleprocessing operations incorporate the polling network technique because of its tremendous advantage over the "conversational mode" in line utilization and computer efficiency.
When a computer or terminal is connected to a modem, which permits it to transmit data over a telephone line, there are specific control leads connected between the modem and the terminal or computer for indicating overall system status. Also connected between the modem and terminal or computer are other leads which are used for protocol between the two units. Such control and other leads include a request to send (RTS) lead through which a local terminal may indicate that data is to be transmitted, a clear to send (CTS) lead by which means the terminal is signalled when the modem is ready to accept data after a RTS signal has been received, a transmitted data lead (TX) for data to be transmitted from the local device to a distant terminal, the local terminal using a transmitted clock (TC) signal to time such data transmission and a received data (RX) lead which contains the data being received by the modem from a distant device for the terminal or computer of the modem, the data on the receive data lead being timed by the receive clock (RC) at the local terminal.
When a plurality of remote terminals are incorporated in a teleprocessing network, one known technique is to associate a modem with each of the terminals and the transmission line, with a modem associated with the computer at the other end of the transmission line. To eliminate the requirement of a modem for each of the remote terminals, one known system incorporates a multiplexer to which all of the remote terminals are connected, the multiplexer being connected in turn to a single modem which is connected over the transmission line to a modem associated with the computer. In a more recently developed system, avoiding certain disadvantages common to the multiplexer arrangement, such as the elimination of multiplexers at both ends of the transmission line, a modem sharer is substituted for one multiplexer. A plurality of terminals are connected to the modem sharer, connected in turn to a modem, the transmission line, another modem and then the computer.
Many such modem sharers are commercially available today, but they are generally characterized by certain disadvantages. For instance, such present day modem sharers are generally subject to short circuits, bringing about defective operation within the various leads in the system, with attendant difficulty in identifying the source of the trouble. In addition, modern day modem sharers do not provide any timing sequence to protect against remote terminals which transmit or lock-up for excessive time periods. Also, such present day modem sharers are limited in the number of remote terminals which they are able to service, imposing a limitation on the versility of such modem sharers.